A fungus nail, which can be other either finger or toe nails, is a unsightly condition and can be very difficult to treat. It usually begins towards the far end of the nail. Starting with patches of white or yellow discoloration, if fungus nail is left untreated, it will proceed to the base of the nail. Then it can even attack the nail root and cause the nail to grow very thick and deformed.
Some people are more susceptible than others because of decreased immunity, abnormal pH levels in the skin and/or trauma to the nail. Most often the big toe is first and then the fungus can spread to adjacent toenails. In rare cases, fungus nail will also affect the skin surrounding the nails. If not treated the nail will become very brittle.
Fungus nail is caused by microscopic organisms called Dermatophytes. These nasty organisms prefer warm, moist conditions such that occur inside shoes, swimming pools, locker rooms and shows. They grow in the nail bed, beneath the nail and live off the protein the nail, known as keratin.
One of the best ways to prevent fungus nail is to keep your feet as clean and dry as possible. Cotton socks will help keep your feet dry because they absorb perspiration. If you have reoccurring fungus nail, an anti-fungal powder may help prevent it. In addition you may want to try:
• Wearing natural cotton socks
• Drying your feet and only using your towel once
• Not sharing your towels or wash cloths with others
• Using paper towels or toilet paper to dry your affected area
• Wearing breathable shoes
Your doctor can also prescribe medication to treat the fungus.

Also, check out this site:  Cure Toenail Fungus